The butterfly was actually developed as a way to swim the breaststroke faster.

The breaststroke has the most restrictive set of rules of any stroke, and is also the slowest stroke. In the 1930s, swimmers began to discover that bringing their arms over the surface of the water, in what we now call the butterfly, was quicker, but still technically met the breaststroke rules. Soon all top swimmers changed to this style. In the 1948 Olympics, the top seven finishers in the 200-meter “breaststroke” actually were doing the butterfly.

Starting with the 1952 Olympics, the rules for breaststroke were tightened, and the butterfly was enshrined as its own event.